In recent years, perovskite solar cells have emerged as a promising technology for renewable energy applications due to their high power conversion efficiencies, tunable bandgaps, and cost-effective fabrication processes. However, one persistent challenge in the solution-based fabrication of perovskite films is the presence of residual PbI2, which can adversely affect device performance by impeding charge carrier transport and increasing non-radiative recombination. In this study, we propose a three-step strategy involving cleaning, passivation, and annealing to address this issue. Our approach utilizes a co-solvent system to effectively remove excess PbI2 while minimizing damage to the perovskite layer, followed by surface passivation with a novel organic ammonium salt and a final annealing step to optimize film morphology and device performance. We demonstrate that this method not only eliminates residual PbI2 but also enhances grain size and crystallinity, leading to a significant improvement in the efficiency of perovskite solar cells.
The performance of perovskite solar cells is highly dependent on the quality of the perovskite absorber layer. Residual PbI2 often remains in the film due to incomplete conversion during the crystallization process, and its presence can lead to insulating regions that hinder electron and hole transport. While some studies suggest that small amounts of PbI2 may passivate grain boundaries, excessive PbI2 is generally detrimental. To overcome this, we developed a co-solvent-based cleaning method that selectively dissolves PbI2 without degrading the perovskite structure. The co-solvent consists of n-butanol and cyclohexane in a volume ratio of 1:4, which provides an optimal balance of polarity to target PbI2 while preserving the perovskite lattice. After cleaning, we introduce a passivation layer using CF3O-PEAI dissolved in a tert-amyl alcohol and cyclohexane mixture (volume ratio 1:4) to fill organic halide vacancies and reduce surface defects. Finally, annealing at 100°C for 5 minutes removes any residual solvents and promotes the formation of a dense, high-quality film.
To evaluate the effectiveness of our strategy, we conducted a series of experiments comparing untreated and treated perovskite films. Scanning electron microscopy (SEM) analysis revealed that the untreated films contained numerous small white crystals identified as PbI2, along with sparse grain boundaries and smaller grain sizes. After co-solvent cleaning, these PbI2 residues were largely removed, and the grain boundaries became more defined, with a slight increase in grain size. Following passivation and annealing, the films exhibited significantly larger grains, reduced grain boundary gaps, and a more compact morphology. This improvement is attributed to the filling of halide vacancies by CF3O-PEAI and the formation of a two-dimensional perovskite layer on the surface, which enhances crystallinity and reduces defect density. X-ray diffraction (XRD) patterns confirmed the reduction of PbI2 peaks after cleaning and the increased intensity of perovskite phases post-treatment, indicating improved crystal quality.
The photovoltaic performance of the devices was characterized through current density-voltage (J-V) measurements under standard AM1.5G illumination. The untreated perovskite solar cells showed a power conversion efficiency (PCE) of 17.93%, with an open-circuit voltage (Voc) of 1.04 V, short-circuit current density (Jsc) of 23.36 mA/cm², and fill factor (FF) of 72.98%. In contrast, the treated devices achieved a PCE of 20.59%, with Voc of 1.08 V, Jsc of 23.62 mA/cm², and FF of 81.60%. This represents a relative improvement of 14.80% in efficiency, primarily due to the reduction of non-radiative recombination and enhanced charge carrier extraction. The increase in Voc is linked to lower defect density, while the improved Jsc and FF result from better interfacial contact and reduced charge transport losses. The efficiency of a perovskite solar cell can be expressed by the equation: $$ \text{PCE} = \frac{J_{sc} \times V_{oc} \times FF}{P_{in}} $$ where Pin is the incident light power density (typically 100 mW/cm² for standard conditions). Our results highlight the critical role of residual PbI2 management in optimizing perovskite solar cell performance.
Further analysis involved examining the charge carrier dynamics and defect states in the perovskite films. We used theoretical models to describe the relationship between defect density and device parameters. The Shockley-Read-Hall recombination theory can be applied to quantify the non-radiative recombination rate: $$ R_{nr} = \frac{n p – n_i^2}{\tau_n (p + p_t) + \tau_p (n + n_t)} $$ where n and p are the electron and hole concentrations, ni is the intrinsic carrier concentration, and τn and τp are the carrier lifetimes. By reducing the defect density through our cleaning and passivation strategy, we effectively increase the carrier lifetimes, leading to higher Voc and FF. Additionally, the grain growth observed in SEM images can be modeled using classical nucleation theory, where the grain size distribution follows a log-normal function: $$ f(d) = \frac{1}{d \sigma \sqrt{2\pi}} \exp\left(-\frac{(\ln d – \mu)^2}{2\sigma^2}\right) $$ where d is the grain diameter, and μ and σ are the mean and standard deviation of the distribution. Our treated samples showed a shift towards larger grain sizes, which correlates with improved charge transport and reduced recombination at grain boundaries.
| Sample | Scan Direction | Voc (V) | Jsc (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|---|
| Untreated | Reverse | 1.04 | 23.36 | 72.98 | 17.93 |
| Untreated | Forward | 1.04 | 23.41 | 73.51 | 17.90 |
| Treated | Reverse | 1.08 | 23.62 | 81.60 | 20.59 |
| Treated | Forward | 1.08 | 23.80 | 78.26 | 20.11 |
The co-solvent cleaning process leverages the solubility differences between PbI2 and the perovskite material. PbI2 has a higher solubility in polar solvents like n-butanol, while the perovskite structure is more stable in less polar environments. The co-solvent mixture achieves a controlled polarity that selectively targets PbI2 without causing excessive dissolution of the perovskite. The volume ratio of 1:4 for n-butanol to cyclohexane was optimized through preliminary experiments to maximize PbI2 removal while minimizing film damage. Similarly, the passivation step uses CF3O-PEAI, which contains trifluoromethoxy groups that enhance its dipole moment and interaction with surface defects. The annealing step not only removes residual solvents but also facilitates the reaction between remaining PbI2 and CF3O-PEAI, forming a thin two-dimensional perovskite layer that passivates the surface and improves moisture resistance. This comprehensive approach addresses both chemical and morphological aspects of film quality, making it highly effective for enhancing perovskite solar cell performance.
In conclusion, our three-step strategy of cleaning, passivation, and annealing using co-solvents successfully removes residual PbI2 from perovskite films and improves their crystallinity and photovoltaic properties. The treated perovskite solar cells exhibit a significant boost in efficiency, from 17.93% to 20.59%, demonstrating the importance of surface management in device optimization. Future work could explore the application of this method to other perovskite compositions and scaling up for module fabrication. Overall, this study provides a robust framework for addressing common challenges in perovskite solar cell development and underscores the potential of co-solvent-based techniques for advancing renewable energy technologies.
To further elaborate on the mechanisms behind our findings, we consider the energy level alignment and interface properties in perovskite solar cells. The presence of residual PbI2 can create energy barriers that impede charge extraction at the interfaces between the perovskite and charge transport layers. By removing PbI2 and passivating the surface, we achieve better band alignment, which facilitates efficient carrier collection. The energy levels can be described using the following relationships: $$ E_c = \chi – \frac{1}{2} E_g $$ where Ec is the conduction band minimum, χ is the electron affinity, and Eg is the bandgap. For our perovskite material, the bandgap is approximately 1.55 eV, and the removal of PbI2 reduces interfacial recombination, as evidenced by the increased Voc. Additionally, the fill factor improvement suggests lower series resistance and higher shunt resistance, which can be modeled using the diode equation: $$ J = J_0 \left( \exp\left(\frac{qV}{nkT}\right) – 1 \right) – J_{ph} $$ where J0 is the reverse saturation current, n is the ideality factor, and Jph is the photocurrent. Our treated devices show a lower ideality factor, indicating reduced recombination losses.
The grain size distribution analysis from SEM images revealed that the treated films had a mean grain size increase of over 50% compared to untreated films. This is critical because larger grains reduce the number of grain boundaries, which are common sites for defect-assisted recombination. The grain growth can be attributed to the passivation step, which supplies additional halide ions to promote Ostwald ripening during annealing. We also observed that the co-solvent cleaning at 50°C accelerated the PbI2 dissolution kinetics, reducing the required processing time from 60 minutes to 20 minutes compared to room-temperature methods. This temperature effect follows the Arrhenius equation: $$ k = A \exp\left(-\frac{E_a}{RT}\right) $$ where k is the rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature. By optimizing the temperature, we achieved efficient PbI2 removal without compromising the perovskite integrity.
| Sample Condition | Mean Grain Size (nm) | Standard Deviation (nm) | Maximum Grain Size (nm) |
|---|---|---|---|
| Untreated | 350 | 150 | 700 |
| After Cleaning | 400 | 120 | 800 |
| After Passivation and Annealing | 550 | 100 | 1000 |
The stability of perovskite solar cells is another important aspect influenced by residual PbI2. Excess PbI2 can decompose under light and heat, leading to the release of iodine vapors and degradation of the perovskite layer. Our passivation with CF3O-PEAI introduces hydrophobic trifluoromethoxy groups that enhance moisture resistance, potentially improving the long-term stability of the devices. We plan to conduct accelerated aging tests in future studies to quantify this effect. In summary, our co-solvent-based approach offers a versatile and efficient method for enhancing the performance and reliability of perovskite solar cells, paving the way for their commercial adoption in solar energy systems.
In terms of material interactions, the co-solvent system works by forming a microemulsion that encapsulates PbI2 particles, allowing for their gentle removal. The polarity of n-butanol (with a dielectric constant of approximately 17.5) interacts with PbI2, while cyclohexane (dielectric constant around 2.0) protects the perovskite matrix. This balance is crucial for maintaining film quality. Furthermore, the CF3O-PEAI passivator not only fills halide vacancies but also forms a dipole layer that repels moisture and reduces ion migration, a common issue in perovskite solar cells. The dipole moment μ can be calculated as: $$ \mu = q \times d $$ where q is the charge and d is the distance between charges. The strong dipole of CF3O-PEAI enhances its passivation efficiency, contributing to the observed performance gains.
Overall, this study underscores the importance of a holistic approach to perovskite film optimization, where residual PbI2 management is combined with surface passivation to achieve high-efficiency solar cells. The methods described here can be adapted to various perovskite compositions and fabrication techniques, making them valuable for the broader field of photovoltaics. As research on perovskite solar cells continues to advance, strategies like ours will play a key role in overcoming material limitations and unlocking the full potential of this technology.